The prior art has recognized the need for thermostatically controlled hydraulic drain valves for draining water systems in buildings and equipment to prevent breakage due to freezing. Typically, these drain valves are powdered by as springtrip mechanism or by an expandable bellows mechanism. Another type of valve considered by the prior art requires a thermostat and a source of external electrical power.
In my prior patent hereinbefore identified, I disclose how unique advantage can be taken of hydraulic pressure to provide a thermostatically controlled valve that is powered by the hydraulic pressure itself and requires no external power or complicated or unreliable springtrip or bellows mechanisms. More particularly, a valve body having an input, output and drain port is provided with a slideable barrier retained within the valve body and suitably adapted to couple the input and output ports and to close the drain port in one position, and to couple the output port to the drain port and close the input in another position. A small coupling orifice which by-passes the barrier allows equal hydraulic pressures to develop on opposite sides of the barrier under static conditions. This orifice acts in combination with thermally sensitive means for relieving the pressure on one side to move the barrier in the direction of the relieved pressure thereby reversing the coupling between the ports.